1. Field of the Invention
The invention relates to an exhaust gas purification device and an exhaust gas purification method for an internal combustion engine.
2. Description of the Related Art
Recently an exhaust gas purification device is making remarkable advancements. However, at cold start, a discharge amount of toxic components, such as HC and NOx, tends to increase since the temperature of the catalyst (e.g. three-way catalyst) is low, and the activity of the catalyst is insufficient.
Using an adsorbent has been proposed as one method to reduce emissions at a cold start. In other words, the toxic components are adsorbed by an adsorbent immediately after the start of the engine until the catalyst reaches the activation temperature. Then after the catalyst is warmed up and activated, the adsorbed toxic component are desorbed from the adsorbent, and purified by the catalyst.
Japanese Patent Application Publication No. 5-256124 (JP-A-5-256124) discloses that if moisture has already been adsorbed in a hydrocarbon adsorbent before the hydrocarbon adsorbent adsorbs hydrocarbon, the hydrocarbon adsorption amount decreases, so a moisture absorbent is added before and after the hydrocarbon adsorbent so as to prevent air containing moisture from flowing into the hydrocarbon adsorbent when an engine is stopping (disclosed in the paragraphs [0003] and [0008] in JP-A-5-256124).
The technology disclosed in JP-A-5-256124 is for preventing the adsorption of moisture in air into the adsorbent when the engine is stopping, however considerable moisture is also contained in exhaust gas. Therefore, it is desirable to suppress moisture from flowing into the adsorbent as well when toxic components are adsorbed into the adsorbent. A possible method to implement this is to dispose a water adsorbent for adsorbing moisture at an upstream side of the toxic component adsorbent.
However, since considerable moisture is contained in the exhaust gas, it is possible that in some cases the water adsorbent becomes saturated during execution of the adsorption operation, and moisture, which could no longer be adsorbed, flows into the toxic component adsorbent. If moisture flows into the toxic component adsorbent, toxic components, once adsorbed, are desorbed from the toxic component adsorbent and flow out, and emission deteriorates.